Difference between a module's config property and directly defining options

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1

I’ve been reading manual on how to define NixOS modules, trying to wrap my head around the inner workings of how the system all comes together. As far as I understand, any file I include with the imports statement, as well as my top-level configuration.nix is referred to as a module. The documentation states that modules can contain an options property to declare options (as in: what types they have) and a config section which then defines actual values for these options (which have been declared in other modules).

Here comes my question: how does this config property differ from simply defining values at the top-level? For example, I’m not sure where the difference is between these two modules:

{ config, ... }: 

{
  environment.etc.firstfile.text = "hello";
  environment.etc.secondfile.text = config.networking.hostName;
}

{ config, ... }: 

{
  config = {
    environment.etc.firstfile.text = "hello";
    environment.etc.secondfile.text = config.networking.hostName;
  };
}

In both cases, I get “hello” in /etc/firstfile and the system’s hostname in /etc/secondfile. If any property that doesn’t have a special meaning gets merged into config anyway, what’s the point of having a special config property? And why do most modules use config instead of directly defining options?

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2

And as a follow-up question: in my second example, which config is line 6 referencing? In my understanding, it should be the config from line 1 and not from line 4, since the latter doesn’t use rec, correct?

3

That’s correct.

As for the difference between putting definitions under config or not, it’s just because doing so is optional if you’re not also declaring options. Just makes config-only files (like a typical beginner’s configuration.nix) a little cleaner looking. Once you want to declare some new options though, you have to put your config definitions under config.

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4

Just want to point it out that if you wanted to reference a value defined inside the config from line 4 (e.g., on line 5) on line 6, you would be able to. While it is correct to say you are using the config from line 1, that one is actually a fixed point created by merging the config in each module, including the one from line 4.

5

Great - thanks for both of your explanations!